Single crystal photovoltaic devices, especially silicon photovoltaic devices have been utilized for some time as sources of electrical power because they are inherently non-polluting, silent and consume no expendable natural resources in their operation. However, the utility of such devices is limited by problems associated with the manufacture thereof. More particularly, single crystal materials are: (1) difficult to produce in sizes substantially larger than several inches in diameter; (2) thicker and heavier then their thin film counterparts; and (3) expensive and time consuming to fabricate.
Recently, considerable efforts have been made to develop processes for depositing large areas of amorphous semiconductor films, which can be doped to form p-type and n-type materials for the production of p-i-n type photovoltaic devices substantially equivalent to those produced by their crystalline counterparts. It is to be noted that the term "amorphous" as used herein, includes all materials or alloys which have long range disorder, although they may have short or intermediate range order or even contain, at times, crystalline inclusions. Also, as used herein, the term "microcrystalline" is defined as a unique class of said amorphous materials characterized by a volume fraction of crystalline inclusions, said volume fraction of inclusions being greater than a threshold value at which the onset of substantial changes in certain key parameters such as electrical conductivity, optical gap and absorption constant occur.
It is of obvious commercial importance to be able to mass produce photovoltaic devices such as solar cells. However, with crystalline cells, mass production was limited to batch processing techniques by the inherent growth requirements of the crystals. Unlike crystalline silicon, amorphous silicon and germanium alloys can be deposited in multiple layers over large area substrates to form semiconductor devices such as solar cells in a high volume, continuous processing system. Such continuous processing systems are disclosed in the following U.S. Pat. Nos. 4,400,409, for "A Method of Making P-Doped Silicon Films And Devices Made Therefrom"; 4,410,588, for "Continuous Amorphous Solar Cell Deposition And Isolation System And Method"; 4,542,711, for "Continuous Systems For Depositing Amorphous Semiconductor Material"; 4,492,181 for "Method And Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells"; and 4,485,125 for "Method And Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells." As disclosed in these patents, the disclosures of which are incorporated herein by reference, a substrate may be continuously advanced through a succession of deposition chambers, wherein each chamber is dedicated to the deposition of a specific semiconductor material. For example, in making a solar cell of n-i-p type configuration, the first chamber is dedicated for depositing an n-type amorphous silicon alloy, the second chamber is dedicated for depositing an intrinsic amorphous silicon alloy, and the third chamber is dedicated for depositing a p-type amorphous silicon alloy.
The layers of thin film semiconductor alloy material thus deposited in the vacuum envelope of the deposition apparatus may be utilized to form a photovoltaic device including one or more p-i-n cells, one or more n-i-p cells, a Schottky barrier, as well as photodiodes, phototransistors, other photosensors, memory arrays, display devices or the like. Additionally, by making multiple passes through the succession of deposition chambers, or by providing an additional array of deposition chambers, multiple stacked photovoltaic cells or other semiconductor devices of various configurations may be fabricated.
The concept of utilizing multiple cells, to enhance photovoltaic device efficiency, was described at least as early as 1955 by E. D. Jackson in U.S. Pat. No. 2,949,498 issued Aug. 16, 1960. The multiple cell structures therein discussed utilized p-n junction crystalline semiconductor devices. Essentially the concept employed different band gap devices to more efficiently collect various portions of the solar spectrum and to increase open circuit voltage (Voc). The tandem cell device (by definition) has two or more cells with the light directed serially through each cell. In the first cell, a large band gap material absorbs only the short wavelength light, while in subsequent cells smaller band gap material absorb the longer wavelengths of light which pass through the first cell. By substantially matching the generated currents from each cell, the overall open circuit voltage is the sum of the open circuit voltage of each cell, while the shirt circuit current thereof remains substantially constant. The photovoltaic device which includes multiple cells, preferably also includes a back reflector for increasing the percentage of incident light reflected from the substrate back through the active semiconductor alloy material of the cells. It should be obvious that the use of a back reflector, by increasing the amount of incident light which is converted to electricity by the semiconductor alloy material, increases the operational efficiency of the photovoltaic device. However, all layers deposited atop the light incident surface of the substrate must be transparent so as to pass a high percentage of incident light from the anti-reflective coating atop the photovoltaic cell to the highly reflective surface of the back reflector from which it is redirected through the semiconductor layers.
The back reflectors formed atop the deposition surface of the substrate if a nontransparent substrate is employed (or atop the body of semiconductor alloy material if a glass substrate is employed) of a photovoltaic device may be either specular or diffuse. With either type of back reflector, light which has initially passed through the active body of semiconductor alloy material of the device, but which is unabsorbed on its initial pass, is redirected by the highly reflective material of the back reflector to pass, once again, through said active body of semiconductor alloy material. The additional pass results in increased photon absorption and charge carrier generation, thereby providing increased short circuit current. In the case of specular back reflectors, wherein the highly reflective material is conformally deposited over a smooth surface, the unused light is generally redirected for one additional pass through the active body of semiconductor alloy material of the device.
Conversely, diffuse back reflectors comprise highly reflective material which is either conformally deposited over a textured surface, or grown in a textured manner upon an underlying surface. Light incident upon said textured back reflector is scattered in addition to being redirected through the active semiconductor alloy material, thereby mandating that a portion of the redirected light travel at angles sufficient to cause said redirected light to be substantially confined within the photovoltaic device, i.e., achieve total internal reflection. Further, since textured, diffuse back reflectors redirect light through the active semiconductor alloy material of the device at an angle, the active semiconductor alloy material can be made thinner than otherwise possible, thereby reducing charge carrier recombination, while still maintaining efficient charge carrier generation and promoting charge carrier collection.
As should be apparent from the foregoing discussion, and since the purpose of a back reflector of a photoresponsive device is to redirect incident light for at least a second pass through the active body of semiconductor alloy material thereof, absorption of that incident light by the back reflector cannot be tolerated. Accordingly, only when very highly reflective, vis-a-vis, absorptive, materials are employed from which to fabricate the back reflector, is the overall efficiency of the photoresponsive device optimized. For use as back reflectors, the most highly reflective material is silver which is characterized by an integrated reflectivity of about 94%. Aluminum is another highly reflective back reflector material commonly suggested from which to fabricate back reflectors, said aluminum material having an integrated reflectivity of about 88%. Yet another highly reflective material which has been proposed for use as a back reflector is copper which is characterized by an integrated reflectivity of about 70%. The last of the most commonly employed reflective materials from which back reflectors are fabricated, is stainless steel having an integrated reflectivity of about 45%. While stainless steel is not nearly as reflective (and indeed, was not described as "highly" reflective) as aluminum, silver, and copper, it has been utilized as a substrate material and hence remains a possible candidate when economic factors are taken into consideration.
Back reflectors commonly formed of the aforementioned highly reflective elements and alloys thereof have been employed in an attempt to provide a suitable light redirecting layer for photoresponsive devices. However, when said highly reflective materials have been employed to improve utilization of incident light, the interdiffusion of the semiconductor alloy material and the highly reflective material was found to be a very significant problem. It was in an attempt to alleviate such diffusion problems (primarily diffusion between elements of the highly reflective material and the body of semiconductor material) that back reflectors formed from such highly reflective materials as aluminum, copper and silver, have been sandwiched between layers of (1) chromium, (2) titanium and titanium oxide, and (3) titanium and tin oxide. However, the use of the adhesion promoting and diffusion inhibiting layers (also referred to as "buffer layers") was not totally effective. The layers had to be deposited to a very thin thickness in order to prevent the layers from absorbing incident light. However, due to their thinness, these layers could not effectively prevent interdiffusion. Alternatively, if the layers were deposited so as to prevent diffusion, the layer would be so thick as to absorb incident light.
For solar cells fabricated on an opaque substrate, following the deposition of the back reflector and buffer layer upon the substrate and the layers of semiconductor alloy material upon the buffer layer, a further deposition process must be performed. In this step a thin, transparent layer of electrically conductive, light transmissive material comprised of, for example, a wide band gap oxide material such as an alloy of indium, tin and oxygen (ITO) or indium doped zinc and oxygen, is deposited atop the layers forming the body of semiconductor alloy material. In the case of photovoltaic cells or photosensors, this transparent, conductive oxide layer forms one of the electrodes thereof. It is the process of continuously and economically depositing such a thin, electrically conductive, light transmissive oxide layer in electrical communication with a body of semiconductor alloy material, to which the present invention is primarily directed.
There are a wide variety of such transparent conductive materials having particular utility in the fabrication of semiconductor devices, such as photovoltaic cells. Generally, materials such as degenerate semiconductors, wide band gap semiconductors, thin metallic films and cermets may be utilized to form the transparent conductive layer. Among some of the specific materials which may be utilized are indium oxide, tin oxide, indium tin oxide, cadmium stannate, zinc oxide, and combinations thereof. While the description herein will primarily be concerned with the deposition of thin, transparent, electrically conductive materials above or below the layers of semiconductor alloy materials as a step in the fabrication of photovoltaic devices, it should be understood that such transparent conductive films also have utility in other electronic devices such as liquid crystal displays, photosensors, light emitting diodes, photochromic, electrochromic devices and the like. Further, such transparent films may also serve other functions in photovoltaic devices, such as, for example, as a buffer layer between said back reflector and said body of semiconductor material.
There are a number of techniques utilized to deposit layers of transparent conductive material in contact with semiconductor bodies. Vacuum evaporation is one such technique. A typical vacuum evaporation process is carried out in a chamber maintained within a pressure regime substantially below atmospheric, typically in the range of 10.sup.-3 to 10.sup.-6 Torr. A charge of the material to be evaporated is placed in a crucible and heated by resistance, induction or electron beam bombardment to produce a vapor thereof, which vapor condenses upon the semiconductor body or other substrate is supported in close proximity to the crucible.
While vacuum evaporation techniques have the advantage of being relatively simple, they are not always well suited for the commercial, high volume preparation of semiconductor devices. First of all, vacuum evaporation processes require relatively low pressures thus necessitating lengthy and complex pump-down procedures. Furthermore, the scale-up of a vacuum evaporation process from a research to a commercial scale, continuous production apparatus is relatively difficult because of the high degree of dependence of the process parameters upon the geometry of the deposition system. It has also been found that in many applications evaporated coatings manifest poor adhesion with the underlying layers upon which they are deposited. And most importantly, evaporation techniques are inherently slow and therefore represent a potential bottleneck in an overall continuous fabrication process.
In contrast to vacuum evaporation, plasma processes such as sputtering, plasma assisted chemical vapor deposition (glow discharge), plasma activated evaporation and the like are fast, easy to control and scale-up, and provide highly adherent coatings. Consequently, they may be advantageously employed in the fabrication of layers of transparent conductive material. In a typical sputtering process, a d.c. or radio frequency signal is employed to generate ions from a working gas maintained at a pressure of typically about 10.sup.-3 torr. Such ions are strongly attracted to, and consequently bombard, an electrically biased target (also referred to as a cathode), thereby ejecting particles of the target material, which particles deposit onto the exposed surface of a substrate maintained in close proximity thereto. In the preparation of a layer of indium tin oxide for example, the face of the target or cathode is a body of solid indium tin oxide material. A working gas, typically argon, is ionized and attracted to the target. The energetic impingement of the argon ions ejects small particles of the indium tin oxide material from the target, which particles condense upon the exposed surface of a substrate.
In a typical glow discharge deposition process, a gaseous reagent is introduced into a low pressure environment and subjected to electromagnetic energy so as to create an activated plasma therefrom. In this plasma, the gaseous reagent mixture reacts to form ionic species which subsequently deposit on a biased substrate maintained within the low pressure environment. In an activated evaporation process, vapor of a precursor material is subjected to an energetic input so as to create a plasma therefrom for facilitating the creation and maintenance of desirable coating species.
As mentioned hereinabove, plasma processes are desirable for use in the deposition of thin layers of transparent, electrically conductive material in the manufacture of electronic devices because such processes are easy to control, can achieve high deposition rates and may be readily scaled-up for large volume production. However, sputter coating or other plasma processes have often been found to result in damage to the underlying body of semiconductor material (or other substrate) upon which the coating material is being deposited. Such damage decreases the efficiency or deleteriously effects other operational parameters of the electronic devices and in some instances can render them completely inoperative. Damage occasioned during the plasma coating of a semiconductor body can result from the energetic impingement of ionic or other energetic species upon that semiconductor body. Such bombardment can, for instance, produce mechanical damage to the semiconductor alloy material, which damage is manifested as broken chemical bonds, vacancies in the matrix of the semiconductor material, and/or over-or-under-coordinated valencies in atoms from which the semiconductor alloy material is formed. In other instances, the ions impinging upon the body of semiconductor alloy material are reactive species themselves and as such partially denature the semiconductor layer; for example, oxygen or nitrogen ions can produce oxide or nitride species in the host matrix of the semiconductor alloy material, thereby dramatically changing its optical, chemical and electrical properties. In other instances, the reactive ions can create an interfacial layer between the depositing material and the body of semiconductor alloy material, thereby limiting electrical contact of the deposited layer with the semiconductor body.
In general, mechanical damage to, or the chemical reaction occasioned by ions or other energetic species depositing upon, the body of semiconductor alloy material produces defect states therein, which defect states adversely affect the operation of the device being fabricated therefrom. For example, it has been found that when a layer of indium tin oxide is sputter deposited onto a body of amorphous silicon or amorphous germanium alloy materials for the fabrication of a photovoltaic device, such silicon germanium body will be damaged due to the energetic sputtering process. In some instances, this damage may be mitigated by employing a very weak and hence very slow rate sputtering process; however, a such a slow process is impractical for purposes of commercial scale production.
The aforementioned plasma techniques have also been employed for the deposition of buffer layers of electrically conductive, wide band gap oxide materials between a body of semiconductor alloy material and the hereinabove discussed back reflector layer. While these techniques have proven to be generally successful for laboratory or batch processing, several areas exist in which improvements may be realized. Specifically, the deposition rates of these materials by the above mentioned techniques is relatively slow, i.e., about 1-5 Angstroms per second. This rate becomes commercially impractical in, for example the deposition of a 5000 angstrom thick buffer layer.
In Applicants' continuous solar cell fabrication apparatus, the layers of semiconductor alloy material are now deposited at about 14 Angstroms/second. It is expected that further improvements in the deposition process would increase that rate to about 18 Angstroms/second within six months. The rate of 1-5 Angstroms/second previously achieved for the deposition of transparent oxide materials therefore represents the bottleneck in the overall solar cell fabrication process. This rate must be improved in order to be able to incorporate the deposition of the transparent oxide material as part of an integrated roll-to-roll fabrication technique.
Additionally, layers deposited by previous activated plasma techniques are exceedingly "smooth" in that they do not enhance the light scattering characteristic of the textured highly reflective back reflector. Indeed, it has been observed that exceedingly smooth layers contribute to specular reflection of light incident upon said back reflector. Even if deposited on diffuse back reflective surfaces, said techniques tend to preferentially deposit in the "valleys" and present a smooth, uniform upper surface.
Another technique which has been explored for depositing layers of wide band gap oxide material is a chemical spray pyrolysis technique, which technique allows for the rapid deposition of relatively high quality material. To date, these techniques have been successfully employed in the batch process fabrication of ZnO/p-CuInSe.sub.2 heterojunction solar cells, as described by M. S. Tomar and M. J. Garcia in Thin Solid Films, 90(1982) 419-423. Further discussion of spray pyrolysis batch deposition of ZnO.sub.x in heterojunction solar cells can be found in an article published in the Journal of Vacuum Science Technology, 16(4), July/August 1979, by J. Aranovich, A. Ortig and R. Bube. These publications discuss spray pyrolysis as being pertinent to the field of heterojunction solar cells prepared by batch processing. However, up to the date of the instant application, there has been no consideration of employing a spray pyrolysis technique for depositing wide band gap oxides, in a continuous manner, for the fabrication of amorphous silicon photovoltaic devices.
Thus, there remains a significant need for a high deposition rate process which will not damage the material upon which it is deposited. Such a process may be readily adapted for the high rate deposition of a wide variety of materials, as a step in the fabrication of photovoltaic devices, photosensors, imaging devices, display devices and other opto-electronic devices. The instant invention thus makes possible, on a continuous basis, the large scale, high yield, commercial scale production of a wide variety of semiconductor devices.
These and other advantages of the instant invention will be apparent from the brief description, the drawings and the detailed description thereof which follow.